Method of making fuel pins by extrusion



1954 w. MARKERT, JR., ETAL 3,160,951

METHOD OF MAKING FUEL PINS BY EXTRUSION Filed Oct. 29, 1957 2 Sheets-Sheet 1 FIG.2

j O- W iTTORNEY 1954 w. MARKERT, JR., ETAL 3,160,951

'METHOD OF MAKING FUEL PINS BY EXTRUSION Filed Oct. 29, 1957 2 Sheets-Sheet 2 FIG.3

INVENTORS 7457/5109 flfarkerz Jr:

BY PazzZC 72 5 9 M wwomn United States Patent "ice 3,160,951 METHOD OF MAKING FUEL PINS BY EXTRUSIQN Wallace Markert, Jr., and Paul C. Thys, Alliance, Ohio, assignors to the Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Oct. 29, 1957, Ser. No. 693,147 11 Claims. (Cl. 29474.3)

This invention relates to the cladding metals which are readily oxidized at elevated temperatures and, more particularly, to a novel method of and apparatus for forming a composite fuel element for an atomic energy reactor comprising a core of essentially fissionable metal integrally bonded to a sheathing of shielding metal.

One type of fuel element used in reactors comprises a core of a high purity uranium alloy, such as U Mo, entirely encased in a shielding metal such as zirconium. The zirconium shield prevents escape of reaction products formed from the uranium alloy. These fuel elements comprise relatively small diameter pins formed by hot working a relatively large diameter billet of the core metal, at least part of the hot working being performed after the core metal is encased in its sheath so that an effective bond is formed between the core and the sheath.

Both the uranium alloy core and the zirconium sheath are readily oxidizable at the elevated temperatures involved in the hot working. In addition, at such elevated temperatures, the zirconium tends to alloy with or gall the work engaging metal surfaces of the hot working apparatus. Consequently, special techniques must be used to prevent such oxidation and galling.

In accordance with the present invention, a fuel element or pin of this type is produced by a method involving a novel sequence of steps, including novel techniques, whereby an improved bond between the core and its sheath are provided and the cost of manufacture of the fuel element is substantially reduced.

More particularly, the core metal, such as U-lO M0, is vacuum melted, so as to obtain a high purity ingot which is sheathed in a protective jacket of oxidized steel or copper and hot rolled, after heating in a furnace to 1750 degrees F, to reduce it to the desired diameter for later cladding with Zr followed by extrusion. The hotreduced billet, after removal of its sheath, is then placed in a closely fitting Zr sleeve slightly longer than the billet, and Zr end pieces or disks are set flush into the sleeve ends against the ends of the billet.

The Zr encased billet is then sealed within a specially prepared carbon steel jacket. It has been found that bonding of the carbon steel to the Zr will occur with the temperature and pressure conditions involved in the extrusion process, making difficult if not possible subsequent removal of the jacket. In accordance with the present invention, such bonding is prevented by oxidizing the carbon steel jacket elements in air for twenty minutes at a temperature of 1600 degrees F. This timetemperature relation is important in that it produces a sufficiently thick oxide film to prevent bonding during extrusion. If longer heating times or high temperatures are used, there is a possibility that the oxide film will blister and fall off the surface thus leaving surface areas with an oxide film too thin to prevent bonding.

The carbon steel jacket comprises a sleeve, a nose piece, and an end d-isk. These parts are assembled around the Zr encased billet and weld united by inert gas shielded metal arc welding in a dry box with an inert gas atmosphere.

An important feature of the invention resides in the method of heating the jacketed and sheathed billet to extrusion temperature in combination with the orientation of the extrusion press. The extrusion press is a vertical press with the ram moving upwardly into the container 3,160,951 Patented Dec. 15, 1964 on the extrusion stroke. The jacketed and sheathed billet is placed in axial alignment on the upper end of the ram with graphite disks or plugs interposed between the lower end of the billet and the ram.

A high frequency induction coil is then placed around the billet and the latter is heated to 1800 degrees F. and held at this temperature for four (4) minutes. The heated billet is then immediately extruded without any repositioning of the parts. The graphite slugs allow comlete extrusion of the billet without discard loss, and this permits a large number of extrusions to be made before re-dressing of the extrusion press dies is necessary. These slugs are removed from the die after each extrusion.

Following the extrusion, the carbon steel jacket is removed either mechanically or by pickling in sulphuric acid. The clad extrusion is then swaged to final diameter and cut into fuel pins of the desired length. The fuel pins may be end capped with Zr in an appropriate manner.

For an understanding of the invention principles, reference is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawing. In the drawing:

FIG. 1 is an exploded view, partly in section, of the core, the sheathing elements, and the outer shield elements;

FIG. 2 is generally an axial sectional view of the core, sheath, and shield as assembled into an extrusion slug; and

FIG. 3 is a part elevation and part sectional view of the induction heating coil, the sing, and par-t of the extrusion press.

The invention will now be described in greater detail with specific reference to the drawings.

The alloy 'of uranium (U) and 10% molybdenum (10 Mo) must be of high purity, and this can be achieved by the use of vacuum melting or arc melting. In the present invention, the alloy ingot is formed by vacuum melting in 21 Zr crucible and by using a graphite mold which is coated with Zr powder to minimize carbon pick-up in the ingot. Excessive carbon pick-up results in the ingot being unsatisfactory for further hot working after casting. In the particular example used by way of illustration, the cast U10 Mo ingot is 1.75" in diameter although larger diameter ingots can be used satisfactorily.

After the ingot is cast, it is heated to 1750 degrees F. in a furnace sheathed in oxidized steel or copper, and then hot rolled to reduce its diameter to 1.0". However, the ingot may be heated in a salt bath before the hot rolling. The resulting billet has its sheath removed and is then machined to 0.810" diameter to form the machined U10 Mo billet 10 of FIG. 1.

The billet is now ready for encasing in Zr and jacketing of the clad billet with carbon steel. Fabrication of the carbon steel jacket may be effected in a variety of ways. By way of example only, a piece 11 of 1" OD. carbon steel tubing is welded to a machined carbon steel nose 12 by helium shielded arc welding. The assembly is then leak tested. The other component of the jacket comprises a machined carbon steel slug 13 which is 0.8650 OD. and 0.250" thick. As stated above, all the carbon steel jacket components are oxidized in air at 1600 degrees F. for twenty (20) minutes to form a bond-inhibiting oxide film.

The Zr cladding components can likewise be fabricated in any desired way provided that good tolerances are maintained. In the specific example selected for illustration, a length of extruded Zr tubing is machined to form a sleeve 14 having an OD. of 0.855" and an ID. of 0.815, the length being greater than that of billet 10 by the combined thickness of the two Zr end caps or wafers 3 15. The latter are punched out of 0.030" thick rolled Zr to a diameter of 0.812.

Since billet and Zr cladding elements 14, have been machined, there is no necessity for pickling or descaling the surfaces and a degreasing with acetone has been found satisfactory. The parts are assembled by placing one Zr end cap 15 and Zr sleeve 14 in the carbon steel sleeve and nose component 11-12, placing billet 10 inside sleeve 14 against the inner end cap 15, placing the second Zr end cap 15 against the outer end of billet 10 and within Zr sleeve 14, and placing carbon steel slug or cap 13 within carbon steel sleeve 11 against outer Zr end cap 15. Slug 13 is then joined to sleeve 11 by helium shielded arc welding in a dry box with a helium atmosphere to form the composite extrusion slug 16 of FIG. 2.

Extrusion is effected in a vertically oriented extrusion press 20, pertinent parts of which are shown in FIG. 3. Press 20 includes a main frame 21 to which is bolted a holder 22 for an outer sleeve or container 23 of a high strength tool steel. Holder 22 has an axial passage 24 therethrough axially aligned with sleeve 23 and with a tubular guide 25 in frame 21 threaded into the upper end of passage 24. The lower end of passage 24 opens into an enlarged recess 26 coaxial with the passage and seating a tool steel bearing plate 27. Container or outer sleeve 23 has mounted therein a tool steel inner sleeve 28 on the upper end of which is seated a tool steel die 30. Container 23 is threaded into a large recess 31 in holder 22 until die abuts bearing plate 27.

Extrusion is eflected by a tool steel ram axially aligned with inner sleeve 28 and movable upwardly thereinto to extrude a billet through die 30. To facilitate entry of the billet into sleeve 28, the lower end thereof is flared as at 32. In the retracted position of ram 35, the upper end of the ram is spaced substantially from the entrance end 32 of sleeve 28 to allow mounting a billet on the ram in axial alignment with sleeve 28.

In accordance With the invention method, jacketed composite billet 16 is placed on the end of ram 35 with graphite disks 33 interposed between the ram and the billet. Sleeve 28, die 30, and ram 35 are then lubricated and a high frequency induction heating coil is then placed co axially around billet or extension slug 16. Slug 16 is induction heated to 1800 degrees F. with the heating continued for four (4) minutes, in the illustrative example, to insure uniform heating of the slug or billet throughout. Ram 35 is then immediately moved rapidly upward through coil 40 to charge slug 16 into sleeve 28 and extrude the slug through die 30. The extrusion takes about two seconds, and is eifected immediately at the end of the heating cycle.

The described heating and extrusion operation has pronounced advantages over one involving a horizontally oriented extrusion press. In the latter case, the extrusion slug or billet is separately heated, as in a furnace, and must then be transferred to the extrusion press. Considerable time is then required to prepare for extrusion, so that there is substantial cooling of the billet before extru sion. To alleviate this cooling elfect, the extrusion press parts have been heated to 1000 degrees F. At this higher temperature, the life of the dies, etc. is substantially reduced.

By contrast, in the present invention where the slug is extruded substantially instantaneously after it is thoroughly heated to the optimum extrusion temperature, the press parts remain relatively cool. This factor, combined with the much shorter heating cycle and use of the graphite slugs 33 providing for complete extrusion of slug 16 without any discard of the slug, greatly reduces the cost of the operation as the die life is increased and the extrusion losses reduced or eliminated. Pick up to metal on the die 30 is greatly reduced so that a relatively large number of extrusions can be made before dressing or replacement of the die is necessary.

The extrusion is removed from guide 25 and the carbon steel jacket 11, 12, 13 removed either mechanically or by pickling in 20% H SO The extreme ends of the extrusion are cut oif, and the extrusion is swaged to a diameter of 0.158 in the illustrative example. Swaging is facilitated by the condition of the extrusion resulting from its prior heating and hot working. The swaged extrusion is then cut into fuel pins of the desired length, such as 30.5" for example, and the fuel pins are end capped with Zr.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the invention principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. The method of fabricating a fuel element for an atomic energy reactor comprising the steps of casting a billet of a uranium alloy; enclosing said billet in a sheath of Zr; forming carbon steel jacketing elements conjointly adapted to have a closely conforming sealing fit around said sheathed billet; heating said jacketing elements in air at a temperature of 1600 degrees F. for substantially twenty (20) minutes to form an adhering oxide film on the surfaces of said jacketing elements; encasing the sheathed billet in said jacketing elements; sealing said jacketing elements to each other; mounting said jacketed and sheathed billet in operative position on the upper end of the vertically movable ram of a vertically oriented extrusion press including a container having an extrusion die at its upper end, the container and die being vertically aligned with the ram and the lower end of the container being spaced from the upper end of the ram in the retracted position of the latter; interposing at least one graphite disk between the lower end of the billet and the upper end of the ram; while said billet is so positioned, heating the same to extrusion temperature by disposing around the billet an induction heating coil, having an inner diameter greater than the diameters of the billet and ram, and energizing said coil with high frequency electrical energy; immediately thereafter extruding the billet vertically upward, to form a relatively elongated and small diameter sheathed and jacketed extrusion having a core of the fissionable metal, by moving the ram and billet upwardly through said coil into the container; and removing the carbon steel jacket from the extrusion.

2. The method claimed in claim 1 in which said uranium alloy is U-lO Mo.

3. The method claimed in claim 1 in which said jacketed and sheathed billet is heated to 1800 degrees F. for substantially four (4) minutes.

4. The method of hot working a metal billet having an exposed metal surface which, at elevated hot working temperatures, is readily oxidizable and easily bonds with contacting metal surfaces; said method comprising the steps of forming carbon steel jacketing elements conjointly adapted to have a closely conforming sealing fit around said billet; heating said jacketing elements in an oxidizing atmosphere at a temperature of 1600 degrees F. for substantially twenty (20) minutes to form an adhering oxide film on the surfaces of said jacketing elements; encasing said billet in said jacketing elements; sealing said jacketing elements to each other; hot working said jacketed billet; and removing the carbon steel jacket from said hot worked billet.

5. The method of fabricating a fuel element for a nuclear reactor comprising the steps of casting a billet of essentially fissionable metal; enclosing said billet in a sheath of shielding metal which, at elevated hot working temperatures, is readily oxidizable and easily bonds with contacting metal surfaces; forming carbon steel jacketing elements conjointly adapted to have a closely conforming sealing fit around said sheathed billet; heating said jacketing elements in an oxidizing atmosphere at a temperature of 1600 degrees F. for substantially twenty (20) minutes to form an adhering oxide film on the surfaces of said jacketing elements; encasing the sheathed billet in said jacketing elements; sealing said jacketing elements to each other; extruding the jacketed sheathed billet to form a relatively elongated and small diameter sheathed and jacketed extrusion having a core of the fissionable metal; and removing the carbon steel jacket from the extrusion.

6. In an extrusion press having a container with a die at one end thereof and an opposite end facing the end of a moveable ram, said ram being aligned with said container and die, the invention comprising the method of extruding a billet comprising the steps of mounting a billet in operative position with one end situated at said end of said ram, spacing said end of said ram from said container a;- distance substantially equal to the axial length of said billet, transmitting heat to said billet while said billet is positioned at the end of said ram, and then extruding said billet through said die by driving said ram toward said container.

7. In an extrusion press having a container with a die at the upper end thereof and the lower end opposite the upper end of a moveable ram, said ram being aligned with said container and die, the invention comprising the method of extruding a billet comprising the steps of mounting a billet in operative position on said upper end of said ram, spacing the upper end of said ram from said container a distance substantially equal to the axial length of said billet, interposing at least one graphite disk between said billet and said ram, transmitting heat to said billet while said billet is positioned at the upper end of said ram, and then extruding said billet upwardly through said die by driving said ram toward said container.

8. In an extrusion press having a container with a die at the upper end thereof and the lower end opposite the upper end of a moveable ram, said ram being aligned with said container and die, the invention comprising the method of extruding a billet comprising the steps of mounting a billet in operative position on said upper end of said ram, spacing the upper end of said ram from said container a distance substantially equal to the axial length of said billet, interposing at least one graphite disk between said billet and said ram, disposing an induction heating coil having an inner diameter greater than the diameters of said billet and said ram around said billet, energizing said coil with high frequency electrical energy and heating directly only said billet to extrusion temperature, and then extruding said billet upwardly through said die by driving said ram toward said container.

9. The method of fabricating a fuel element for a nuclear reactor comprising the steps of casting a billet having a fissionable isotopic content; enclosing said billet in a sheath of shielding metal which, at elevated hot working temperatures, is readily oxidizable and easily bonds with contacting metal surfaces; forming carbon steel jacketing elements conjointly adapted to have a closely conforming sealing fit around said sheathed billet; heating said jacketing elements in an oxidizing atmosphere at a temperature of 1600 degrees F. for substantially twenty (20) minutes to form an adhering oxide film on the surfaces of said jacketing elements; encasing the sheathed billet in said jacketing elements; sealing said jacketing elements to each other; extruding the jacketed sheathed billet to form a relatively elongated and small diameter sheathed and jacketed extrusion having a core with a fissionable isotopic content and removing the carbon steel jacket from the extrusion.

10. A method of fabricating a fuel element containing fissionable material for a nuclear reactor comprising the steps of enclosing said fissionable material in a sheath of shielding metal which is readily oxidizable and easily bonds with contacting metal surfaces at elevated hot working temperatures, forming a carbon steel jacketing element adapted to have a closely conforming sealing fit around said sheath, heating said jacketing element in an for a time sufficient to form a substantially uniform adhering oxide film on its surface adapted to contact with oxidizing atmosphere to a predetermined temperature and said sheath, sealingly enclosing said sheathed material in said jacketing element, hot working the jacketed sheathed material to form a sheathed and jacketed member having a core of said fissionable material, and removing the carbon steel jacketing element from the Worked sheathed material.

11. A method of fabricating a fuel element containing fissionable material for a nuclear reactor comprising the steps of enclosing said fissionable material in a sheath of shielding metal which is readily oxidizable and easily bonds with contacting metal surfaces at elevated hot working temperatures, forming a carbon steel jacketing element adapted to have a closely conforming sealing fit around said sheath, heating said jacketing element in an oxidizing atmosphere to a predetermined temperature and for a time suflicient to form a substantially uniform adhering oxide film on its surface adapted to contact with said sheath, sealingly enclosing said sheathed material in said jacketing element, extruding the jacketed sheathed material to form a relatively elongated sheathed and jacketed member having a core of said fissionable material, and removing the carbon steel jacketing element from the worked sheathed material.

References Cited in the file of this patent UNITED STATES PATENTS 567,410 Potter Sept. 8, 1896 2,075,622 Nehlsen Mar. 30, 1937 2,358,892 Upton Sept. 26, 1944 2,653,494 Creutz Sept. 29, 1953 2,806,596 Dodds et al Sept. 17, 1957 2,820,751 Saller Jan. 21, 1958 2,870,907 Creutz Jan. 27, 1959 2,875,311 Harkenrider Feb. 24, 1959 2,893,555 Buffet et al. July 7, 1959 OTHER REFERENCES International Conference on the Peaceful Uses of Atomic Energy, 1955, vol. 9, page 222.

Nuclear Power, June 1956, vol. 1, No. 2, pages 82-88.

Nuclear Power, July 1956, vol. 1, No. 3, pages 129.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 l6O 95l December 15 1964 Wallace Markert Jr., et alu It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l line 52 for "possible" read impossible column 3 line 71, for "to" read of --5 column 6, line l8 strike out "oxidizing atmosphere to a predetermined temperature and" and insert the same after line 15, same column 6,

Signed and sealed this 27th day of April 1965c (SEAL) Attest:

ERNEST W. SWIDER I EDWARD J. BRENNER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 160 951 December l5 1964 Wallace Markert Jr, H et a1 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1 line 52 for "possible" read impossible column 3 line 71 for "to" read of g column 6 line l8 strike out "oxidizing atmosphere to a predetermined temperature and" and insert the same after line 15, same column 6.,

Signed and sealed this 27th day of April 19650 (SEAL) Attest:

ERNEST W. SWIDER I EDWARD J. BRENNER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 160 951 December 15 1964 Wallace Markert Jr, 9 et a1 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1,, line 52 for "possible" read impossible column 3 line 71, for "to" read of column 6 line l8 strike out "oxidizing atmosphere to a predetermined temperature and" and insert the same after line l5 same column 6 Signed and sealed this 27th day of April 1965,

(SEAL) Attest:

ERNEST W. SWIDER' I EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

6. IN AN EXTRUSION PRESS HAVING A CONTAINER WITH A DIE AT ONE END THEREOF AND AN OPPOSITE END FACING THE END OF A MOVEABLE RAM, SAID RAM BEING ALIGNED WITH SAID CONTAINER AND DIE, THE INVENTION COMPRISING THE METHOD OF EXTRUDING A BILLET COMPRISING THE STEPS OF MOUNTING A BILLET IN OPERATIVE POSITION WITH ONE END SITUATED AT SAID END OF SAID RAM, SPACING SAID END OF SAID RAM FROM SAID CONTAINER A DISTANCE SUBSTANTIALLY EQUAL TO THE AXIAL LENGTH OF SAID BILLET, TRANSMITTING HEAT TO SAID BILLET WHILE SAID BILLET IS POSITIONED AT THE END OF SAID RAM, AND THEN EXTRUDING SAID BILLET THROUGH SAID DIE BY DRIVING SAID RAM TOWARD SAID CONTAINER.
 10. A METHOD OF FABRICATING A FUEL ELEMENT CONTAINING FISSIONABLE MATERIAL A NUCLEAR REACTOR COMPRISING THE STEPS OF ENCLOSING SAID FISSIONABLE MATERIAL IN A SHEATH OF SHIELDING METAL WHICH IS READILY OXIDIZABLE AND EASILY BONDS WITH CONTACTING METAL SURFACE AT ELEVATED HOT WORKING TEMPERATURES, FORMING A CARBON STEEL JACKETING ELEMENT ADAPTED TO HAVE A CLOSELY CONFORMING SEALING FIT AROUND SAID SHEATH, HEATING SAID JACKETING ELEMENT IN AN FOR A TIME SUFFICIENT TO FORM A SUBSTANTIALLY UNIFORM ADHERING OXIDE FILM ON ITS SURFACE ADAPTED TO CONTACT WITH OXIDIZING ATMOSPHERE TO A PREDETERMINED TEMPERATURE AND SAID SHEATH, SEALINGLY ENCLOSING SAID SHEATHED MATERIAL IN SAID JACKETING ELEMENT, HOT WORKING THE JACKETED SHEATHED MATERIAL TO FORM A SHEATHED AND JACKETED MEMBER HAVING A CORE OF SAID FISSIONABLE MATERIAL, AND RE MOVING THE CARBON STEEL JACKETING ELEMENT FROM THE WORKED SHEATHED MATERIAL. 